This invention relates to an acceleration micro-sensor which is made of a semiconductor.
A conventional semiconductor acceleration sensor is shown in FIGS. 12A and 12B. FIG. 12A is a perspective view and FIG. 12B is a circuit diagram. In FIG. 12A, the semiconductor acceleration sensor comprises a quadrangular prism-like thick weight 10 made of a semiconductor and having a thickness of, for example, 400 microns, a thick support member 12 which is separated by a predetermined gap from the weight 10 and formed so as to surround it, and a thin beam 11 which connects one side face of the weight 10 with that of the support-member 12 facing the one side face and which has a thickness of, for example, 10 to 40 microns. Strain gauges 31, 32, 33 and 34 are formed on the upper face of the beam 11. The strain gauges 31 and 33 are formed in the side of connecting it with the weight 10 and in the longitudinal direction of the beam 11, and the strain gauges 32 and 34 are formed in the width direction of the beam 11. These strain gauges 31 to 34 are electrically connected as shown in FIG. 12B to constitute a Wheatstone bridge in which the strain gauges 31 and 33 are opposite to each other and the strain gauges 32 and 34 are opposite to each other. In FIG. 12B, V designates a power supply terminal, and S.sub.1 and S.sub.2 designate signal output terminals.
When acceleration in the vertical direction (which is the direction of detecting acceleration) is applied to the weight 10, the weight 10 is subjected to a force in the vertical direction and the beam 11 deflects in the direction indicated by arrow P. At this time, a tensile stress acts on the upper face of the beam 11, so that the resistance of each of the strain gauges 31 and 33 formed in the longitudinal direction of the beam 11 increases, and, in contrast, the resistance of each of the strain gauges 32 and 34 formed in the width direction of the beam 11 does not change. This causes a detection signal the level of which is proportional to the magnitude of the acceleration, to be output from the signal output terminals S.sub.1 and S.sub.2 of the Wheatstone bridge.
Because of the configuration where the weight 10 is supported at only one end, the semiconductor acceleration sensor has an impaired impact strength. As shown in FIG. 13, therefore, such a semiconductor acceleration sensor is usually accommodated in a hermetically sealed container 850 which contains a damping liquid 830. In FIG. 13, 800 designates the semiconductor acceleration sensor and 820 designates an amplifier for a detection signal.
In the semiconductor acceleration sensor mentioned above, as shown in FIG. 14, the deflection center line 13 of the beam 11 is separated by a distance L from the center of gravity G of the weight 10. When acceleration in the transverse direction (which is the direction of not detecting acceleration) is applied to the weight 10, therefore, a moment indicated by arrow M is generated by this acceleration and the distance L to be applied to the weight 10 so that the weight 10 is subjected to a force in the vertical direction in the same manner as the case where acceleration in the vertical direction is applied, thereby deflecting the beam 11 in the direction indicated by arrow P. This deflection causes the Wheatstone bridge to output a signal, and this signal output functions as an interference output to impair the detection accuracy.
As a countermeasure to this problem, a configuration may be proposed in which, as shown in FIG. 15, an additional weight 14 made of glass or the like is jointed to the upper face of the weight 10 so that the center of gravity G of the weight consisting of the weight 10 and the additional weight 14 exists on the deflection center line 13 of the beam 11, thereby reducing the distance L therebetween to zero. However, this improved configuration has a problem in that an extra process step of joining the additional weight is required .and the production cost is increased.
In the production of a semiconductor acceleration sensor such as that shown in FIG. 12 or 15, the weight 10, the support member 12 and the beam 11 are formed by engraving both the upper and lower faces of a semiconductor substrate using working means such as a plasma etching apparatus. In such a plasma etching process, because of its working characteristics, the etching proceeds at a high rate when the working width is large and proceeds at a low rate when the working width is small. In the case that different working widths exist in a semiconductor substrate under the working process as indicated by W.sub.3 and W.sub.4 in FIG. 16, therefore, different engraving depths are obtained as indicated by D.sub.3 and D.sub.4. This raises a problem in that the accuracy of the engraving process is lowered, thereby reducing the production yield.
In order to improve the impact resistance, the above-mentioned semiconductor acceleration sensors are usually accommodated in a hermetically sealed container which contains a damping liquid. The existence of the damping liquid causes the detection sensitivity to be reduced, and therefore it is required to estimate the reduction rate and to adjust the sensitivity before introducing the damping liquid into the container. Since the viscosity and compressibility of the damping liquid change depending on the pressure and temperature, however, sensitivities vary to arise a further problem in that the production yield is impaired.
Furthermore, FIGS. 22 and 23 illustrate another conventional semiconductor acceleration sensor by way of example: FIG. 22 is a top view and FIG. 23 a side view. As shown in FIGS. 22 and 23, the semiconductor acceleration sensor comprises a thick-walled square weight 901 which is, for example, 400 microns thick, a thick-walled square support 906 set a predetermined space apart from one side of the weight, and a thin-walled beam 907 which is, for example, 20 to 40 microns thick, the beam coupling the one side of the weight 901 and an opposed side of the support 906. Strain gauges 907A, 907B, 907C, 907D are formed in the beam 907. The stain gauges 907A, 907C among them are formed in the top surface of the junction between the beam 907 and the support 906 in the lengthwise direction of the beam 907, whereas the strain gauges 907B, 907D are formed in the top surface of the junction between the beam 907 and the weight 901 in the crosswise direction of the beam 907. Further, these strain gauges 907A, 907B, 907C, 907D are used to form the Wheatstone bridge by respectively setting the strain gauges 907A, 907C, and those 907B, 907D to face each other as shown in FIG. 26. In this case, E denotes a power supply terminal, G a ground terminal, and S1, S2 signal output terminals.
When acceleration is applied to the weight 901 in direction of arrow V in FIG. 23, that is, in a direction perpendicular to the weight 901 (the direction in which the acceleration is detected), the weight 901 receives vertical force Fv, thus causing the beam to bend down in direction of arrow M as shown in FIG. 27. At this time, tensile stress acts on the top surface of the junction between the beam 907 and the support 906 and that of the junction between the beam 907 and the weight 901. As a result, the resistances of the strain gages 907A, 907C formed in the lengthwise direction of the beam 907 increase, whereas those of the strain gauges 907B, 907D formed in the crosswise direction remain unchanged. Detection signals whose strength is proportional to the acceleration are thus output from the output terminals S1, S2 of the Wheatstone bridge.
As an ordinary diffusion technique is used for forming the strain gauges 907A, 907B, 907C, 907D, the surfaces of the weight 901, the beam 907 and the support 906 are covered with a passivation film 910 of SiO2, SiN or the like to protect them.
Since there exists a distance L from the strain centerline 909 of the beam 907 to the center of gravity W of the weight 901 in the acceleration sensor of FIG. 23, the distance L from the strain centerline 909 to the center of gravity W of the weight 901, and the crosswise force Fh produced in the weight 901 due to acceleration causes a moment when the acceleration is applied to the weight 901 crosswise (the direction in which acceleration is non-detected) as shown by an arrow H. Consequently, the beam 907 is caused to bend down in direction of arrow M as in a case where acceleration is applied vertically to the beam 907. In response to the strain, the Wheatstone bridge outputs a signal, which makes an interference output and lowers detection accuracy.
For the reason stated above, it may be considered remedial to reduce the distance L to zero by joining an additional weight 908 such as glass to the top surface of the weight 901 in order to make the center of gravity W of the combination of the weight 901 and the additional one 908 conform to the strain centerline 909 of the beam 907; however, the additional process step of joining them will increase the cost further.
Another problem arising from the aforementioned acceleration sensor is that detection sensitivity is low since the strain gauges formed in the top surface of the junction between the beam and the weight in the crosswise direction of the beam produce no resistance changes when acceleration is applied.
Moreover, the passivation film of SiO2, SiN or the like for the protection of the strain gauges is normally processed at high temperatures before being put back to the normal temperature. Notwithstanding, the difference in thermal expansion coefficient between the passivation film and the silicon semiconductor may cause the bending of the beam 907 as shown in FIG. 29 because of the stress generated on the surface of the silicon semiconductor when the normal temperature is restored. The situation in which acceleration has been applied is brought about when the beam is caused to bend and voltage is output from the Wheatstone bridge likewise. This voltage is called an offset output and lowers not only the S N ratio of the sensor output but also detection accuracy.